1. Field of the Invention
This invention relates to pipe-cutting tools for cutting pipe within a well bore, and more particularly, to an exterior, or overshot, pipe cutter.
2. Description of the Prior Art
During operations associated with the extraction of hydrocarbons from producing formations it sometimes occurs that a portion of the drill string that has been axially advanced to generate the well bore may become engaged within the bore. When this occurs, retraction or manipulation of the string is prohibited or encumbered. Since the piping comprising the drill string is relatively expensive, it is desirable and economically advantageous to recover as much of the string as possible from the bore.
For this purpose tools known in the art as outside, or overshot, pipe cutters are utilized. Such overshot cutters carry radially inwardly extendable cutting elements. Upon actuation, the cutting elements engage the exterior surface of the drill string to be severed (commonly known as the fish), and sever the string from the outside thereof. Upon a completion of the cut, the severed portion of the fish rests upon the cutting elements, which thereby provide supports for carrying it to the surface with retraction of the cutter.
Due to the close radial clearances between the exterior surface of the drill string and the interior surface of the bore, it is necessary that all operative portions of the overshot cutter, including the cutting elements and the actuating mechanism therefor, be initially housed within a relatively thin-walled, annular structure. The structure is lowered into the annular space between the exterior of the drill string and the interior of the bore until the cutting elements are in proximity to the point at which it is desired to sever the fish. Usually the actuating mechanism comprises inner and outer concentric, relatively movable, members which are initially joined together by a shear pin of easily severable material. This joining of the concentric inner and outer members prevents premature actuation and extension of the cutting elements and yet permits expeditious actuation and extention thereof when it is desired to do so. However, some form of gripping element is usually provided to enable the inner member to remain stationary with respect to the outer member to permit severance of the shear pin.
The sections of the drill string engaged within the well usually comprise elongated axial portions of pipe joined to the next-axially adjacent portion at each end thereof by a radially distended collar. These periodic radial distensions of the collars between adjacent drill pipe sections obstruct the dimension of the annulus between the exterior of the string and interior of the bore and require that some provision be made in the overshot cutter to permit the gripping elements to pass in an axially downstring direction over the distended collars.
It may, therefore, be appreciated that it is necessary for an overshot cutter to exhibit an effective annular dimension sufficient to permit it to pass substantially unhampered over the radially distended collar portions of the drill string as the overshot is moved axially downstring. Yet, at the same time, it is necessary that the cutter provide a radially inwardly disposed gripping arrangement such that proper bracing may be effected between the concentric inner and outer members to permit severance of the shear pin and extension of the cutter elements.
A common method of meeting both these goals is to provide resilient spring steel members cantilevered about the inner surface of the overshot cutter. These members serve as gripping elements able to bend or flex radially outwardly as the overshot moves in the downstring direction. Such spring steel members can, therefore pass substantially unhampered over the radially distended collars as the cutter is moved in a downstring direction. However, once axially cleared of the collars, the spring steel gripping members are resiliently urged back into contact with the smaller diameter of the elongated pipe sections and maintain contact therewith. Thus, when it is desired to sever the shear pin and extend the cutting elements, an axially upstring movement imparted to the overshot brings the gripping elements in contact with the underside of the collars. The gripping contact therebetween provides suitable bracing to permit severing of the shear pin. U.S. Pat. Nos. 1,458,118 (Plante), 1,709,141 (McCullough), 1,855,338 (Church), and 1,936,643 (Reed), all disclose outside pipe cutting apparatus embodying resilient, cantilevered gripping elements operable as described.
One disadvantage of the cutters using the structure of the last-mentioned patents is that once the spring steel gripping elements have passed over the radial obstruction in the axial downstring direction, the only way to retract the overshot cutter before making the cut is to apply sufficient axially upward force to sever both the gripping elements and the cutting elements. This is disadvantageous.
It has also been the practice in the prior art to utilize an overshot cutter which provides constant annular dimension throughout its entire length. Such structures do not have radially inwardly extending resilient gripping elements, or the like. Thus, the problem of movement over the distended collars is not presented. Instead, complicated structural elements are provided which serve as radially inwardly extendable gripping elements once the cutter is in position. Cutters of this type thus include a two-step operative mode. The first step, when the cutting elements are in proximity to the portion of the string to be severed, requires extension of the gripping elements radially inwardly to engage the fish to be severed. The second step of the operation is the utilization of the gripping elements to provide the radially inwardly extension of the cutting elements to contact the pipe to be cut. Exemplary of such prior art devices are U.S. Pat. Nos. 1,454,819 (Jones et al.), 1,944,594 (Church et al.), 2,160,691 (Edwards), 3,031,015 (DeCordova), and 3,056,453 (Foreman, Jr.).
Such cutters have the disadvantage of being overly complicated from the structural standpoint, and, for this reason, are disadvantageous.
It is also known in other areas of oil field technology, such as in latch assemblies for the gripping of a fish from the interior thereof, to provide flexible sleeves which are locked and prevented from radial movement by a radially inwardly disposed locking member. Upon axial displacement of the locking member, sufficient radial clearance is provided to permit radial movement of the sleeve. For example, U.S. Pat. Nos. 3,638,988 (Brown), 3,874,447 and 3,874,448 (both to McGowan, Jr.) exhibit radially movable members initially prevented from radial displacement by the provision of a locking element. Upon axial movement of the locking element sufficient radial clearance is provided to permit radial displacement of the movable members.
In the context of overshot cutting elements, it is desirable to provide an overshot cutter having radially inwardly extending gripping elements which engage a portion of the pipe to be severed to thereby brace an inner member of the overshot with respect to an outer concentric member thereof to actuate and to extend the cutting elements into engagement with the pipe to be severed. It is also advantageous to provide an overshot cutter able to permit radial displacement of the gripping elements as those gripping elements become obstructed during downstring movement of the overshot into the well bore. For this purpose it is advantageous to provide an overshot cutter having an inner member including a flexible portion with inclined surfaces adopted to be received and engaged by correspondingly inclined surfaces on the outer member so that abutment of the surfaces prevents radial movement of the flexible inner member. However, it is advantageous that when the inclined surfaces are disengaged and spaced axially apart, a radial clearance is defined sufficient to permit the inner member and the gripping elements thereon to flex radially outward to permit the gripping elements to clear the radial distensions on the exterior surface of the drill string.
It is of even further advantage to provide an overshot cutter wherein one embodiment thereof is arranged so that a spring or other suitable biasing element maintains the inclined surfaces in engagement while the overshot is moving unhampered and unobstructed in a downstring direction. Yet, when the inner portion and gripping elements become obstructed by radially distended portions of the drill string, continued axial displacement of the outer member spaces the inclined surfaces apart against the bias of the spring to provide sufficient radial clearance to permit the inner member and the gripping elements to move radially outwardly and pass the obstruction. It is also of advantage to provide a second embodiment of the overshot cutter such that rotation of the outer member places a radially inwardly extending dowel thereon into a groove provided on the concentric inner member. Engagement of the dowel with the groove axially spaces apart the inclined surfaces and provides sufficient radial clearance to permit the gripping elements to move radially outwardly from the obstruction.